1. Field of the Invention
The invention relates to a new process for preparing imidazopyridine derivatives of the general formula: ##STR2## wherein R.sub.1 is hydrogen or an alkyl group, and R.sub.2, R.sub.3 and R.sub.4 are identical or different and are hydrogen, an alkyl group, a cycloalkyl group, an aryl group, an aralkyl group or a halogen atom.
2. Background Art
According to European Published Patent Application No. 0456510, 5,7-dimethyl-2-ethylimidazo[4,5b]pyridine is obtained in admixture with 4,6-dimethyl-2,5-bis(propionamido)pyridine by hydrogenation of an isomer mixture of 2-amino-3-nitro-4,6dimethylpyridine and 2-amino-5-nitro-4,6-dimethylpyridine using a palladium catalyst and hydrogen, by subsequent isolation of the resulting mixture of 2,3-diamino and 2,5-diamino isomers and by condensation of this isomer mixture with propionic acid in the presence of polyphosphoric acid. The purification of the desired imidazopyridine is carried out by column chromatography. The isomer mixture of the aminonitropyridines used in the process of European Published patent Application No. 0456510 is obtained by nitration of the corresponding aminopyridine. A considerable disadvantage of this known synthesis is that, over the whole process, an undesired isomer is also reacted which finally results in the end product containing a by-product which is difficult to separate off. The reaction conditions, in particular the long hydrogenation time of 18 hours, likewise make the process unattractive for use on an industrial scale.